Flying at FL270, you experience sever turbulence and Mode C failure, and the ATC channel is heavily Occupied, what should a pilot ideally do?
In case of imminent danger remember these three words: aviate, navigate communicate. In that order.
- Aviate: make sure you keep flying the aircraft, and keep it flying in one piece.
- Navigate: know what is around you and control where you are going. Avoid crashing into anything or anyone else.
- Communicate: let ATC and/or other aircraft know what your intentions are.
If we apply that to this situation:
- Aviate: Immediately reduce speed to maximum turbulence penetration speed Vb or, when published for your aircraft, the optimal turbulence penetration speed Vra. Control your airspeed, don't stall. Control your attitude; keep up up and down down.
- Navigate: Identify whether a deviation of your clearance would improve the turbulence situation (e.g. because you flew into turbulence caused by a Cumulonimbus, turning / climbing may get you clear sooner). Now that you have identified a way out, think about the implications: being in Cumulonimbus is bad, but flying into Cumulus Granitus (a.k.a. mountain) is worse. Are you aware of all terrain / man made obstacles around you? Is there other traffic? (abuse the TCAS display for this, if outside of cloud do a visual scan). Especially vertical deviations (common in severe turbulence) of your clearance may bring you into traffic from the opposite direction. Consider a lateral 45 degree deviation of your path (terrain / weather permitting). And don't forget 1.: keep flying the aircraft.
- Communicate: Only when 1. and 2. are under control and as soon as you get the chance, let ATC know what you are doing and why. Oh, and don't forget 1. and 2.: keep flying the aircraft and don't fly into something.